>-----Original Message-----
>From: spam_OUTpiclist-bouncesTakeThisOuTmit.edu [.....piclist-bouncesKILLspam@spam@mit.edu]
>Sent: 14 December 2005 16:00
>To: Microcontroller discussion list - Public.
>Subject: [EE:] Detecting a blown lamp
>
>
>Hi Everyone,
>
>I'm making a little PIC-controlled floodlight interface for my
>house, and I'd like the PIC to be able to detect if the lamp
>filament is working.
>
>The PIC is running off a 7805 from a wall-adapter, and is
>happily controlling the floodlight via a triac. The floodlight
>is running off a 240vAC fused spur and is rated for 500W. I
>have another floodlight I'd like to use for a second interface
>that is 250W, so I'd like to be able to create a 'generic'
>broken-filament detector that doesn't rely on there being a
>specific wattage of bulb.
>
>I've seen the X-10 Microchip app note where they just use a
>mega Ohm resistor to connect a PIC pin up to the mains for the
>zero-crossing detect, but I'm not sure on the safety issues of
>that one.
>
>My other idea was a transistor with it's base connected via a
>suitably sized resistor to the 240v live just before the
>floodlight. The transistor would drive an optoisolator. I've
>seen something similar on piclist.com describing a stalled
>motor detector, but I've no idea if it would work with a lamp
>of variable wattage at 240vAC.
>
>Any thoughts/ideas appreciated.
>
>Cheers,
>
>Tim

A simple current transformer would be one of the safer methods, with total isolation from 240v. The output of the transformer would need to be rectified and the voltage level suitably clamped for the PICs inputs.

Regards

Mike

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